How Do Capacitive Sensors Work
The rise of the smartphone has made capacitive sensing ubiquitous and placed this technology in the hands of practically every person. Although most people don’t know it, the touch screen of smartphones uses capacitive sensors. Capacitive sensors make use of the capacitor, a common element in an electrical circuit, of which there are several different kinds. At Bare Conductive we use capacitive sensing in our hardware, such as the Touch Board, Pi Cap and Light Up Board. In these boards, each electrode acts as a capacitive sensor.
In an electrical circuit, you often find a resistor and a capacitor, the resistor is a passive element in the circuit and the capacitor an active one. A capacitor in its common form consists of two parallel plates that are separated by a dielectric material, such as plastic, glass or porcelain. When a voltage is applied across the plates, a charge accumulates on each plate, creating an electric field across the plates. The amount of charge a capacitor can store, known as the capacitance, depends on the size of the plates, the distance between them and the dielectric constant of the material between them. Capacitors can be used to smooth out the output voltage of a power supply or for filtering in signal processing.
A capacitive sensor uses the characteristics of a capacitor and its electrical field to form a sensor. By detecting any change in the electric field the sensor can register either touch or proximity, displacement, as well as the level detection of humidity and fluids.
There are two common ways to use capacitance sensing, these are mutual capacitance and self-capacitance. In mutual capacitance, there is an array of one plate of capacitors, which all form electrical fields between them. This is the kind of sensor set-up you find a touch screen. When the user touches the screen, he or she changes the capacitance, which is detected as a change and interpreted as a touch event.
In self-capacitance, which is what we use in our technology, an electrode forms a capacitor with the ground plane, which is the earth. So when you turn one of our devices on, it does some calibration and forms an electrical field with the earth. When the user approaches the sensor electrode with their finger or hand, their body part takes over from the earth and form the second capacitor plate. Again, this change in the electric field changes the capacitance of the sensor and is registered as a touch or proximity event.
The capacitive sensing technology on our hardware can either be used as a capacitive proximity sensor or as a capacitive touch sensor and can be extended with a conductive material, like our Electric Paint. The capacitance of a sensor can be changed by either an organism or fluids, so these sensors can also be used for humidity or fluid detection and can detect change through a layer of bulk material, so you can create a discrete sensor within your environment. The sensing distance of a proximity sensor depends on the size of the sensor, the design of the sensing area, the sensitivity of the device and the target size. Just to give you a figure, our sensors can detect a hand with a 20cm wide sensor from about 5cm.
There are other sensors, such as the capacitive displacement sensor or position sensor. These kinds of sensors are used more commonly in manufacturing, where a change in capacitance is used to detect the position and displacement in an assembly.
If you want to explore capacitive sensors in your own applications, our development kits and hardware provide an easy starting point for prototyping and testing how capacitive sensing can work for different applications, with different materials, and at different scales. For companies looking to integrate capacitive sensors directly into their solutions, materials or products, we are always looking for interesting partnerships where our technology can add value.
We believe our technology holds the promise of truly integrating the Internet of Things into our lives in a seamless and non-intrusive way through printed electronics on smart surfaces, and we’re working with industry leading partners to build capacitive sensors directly into the environment making the spaces and surfaces around us smarter, healthier, and more extraordinary.
To explore our development kits and materials, check out our online shop.
For more information on how we we work with industry to develop smart surface solutions go to our technology page.